Two novel types of l-glutamate receptors with affinities for NMDA and l-cysteine in the olfactory organ of the Caribbean spiny lobster Panulirus argus

Abstract

A subset of olfactory receptor neurons of the Caribbean spiny lobster Panulirus argus possesses receptors for l-glutamate that can mediate both excitatory and inhibitory responses (P.C. Daniel, M.F. Burgess, C.D. Derby, Responses of olfactory receptor neurons in the spiny lobster to binary mixtures are predictable using a non-competitive model that incorporates excitatory and inhibitory transduction pathways, J. Comp. Physiol. A 178 (1992) 523–536). In this study, we have used biochemical and electrophysiological techniques to understand the role of these receptors in olfactory transduction, and to compare these olfactory glutamate receptors with peripheral and central l-glutamate receptors in other animals. Using a radioligand-binding assay with a membrane-rich preparation from the dendrites of olfactory receptor neurons, we have identified two types of binding sites for l-glutamate. Both sites showed rapid, reversible, and saturable association with radiolabeled l-glutamate, and their K d values (1 nM and 3 μM) are effective in physiological studies of glutamate-sensitive olfactory neurons, suggesting these binding sites are receptors involved in olfactory transduction. Both sites were completely inhibited by high concentrations of NMDA and l-cysteine, and only partially inhibited by other l-glutamate analogs and odorants. Electrophysiological recordings from l-glutamate-best olfactory receptor neurons showed that NMDA and l-cysteine are both partial agonists and antagonists of glutamate receptors. Together, these results suggest the olfactory l-glutamate receptors of spiny lobsters are novel types of l-glutamate receptors that are functionally important in mediating olfactory responses.